In the era of advanced information technology of the 21st century, a great deal of information should be obtained promptly with ease, and thus an importance of the high performance flat panel display for multimedia increases. Although liquid crystal displays (LCDs) have played the main part of flat panel displays up to now, many attempts are made to develop novel flat panel displays that are cost-efficient, show excellent performance and are differentiated from liquid crystal displays all over the world. Organic electroluminescence (EL) devices or organic light emitting devices that are expected to play an important role as advanced flat panel displays have advantages of lower drive voltage, higher response rate, higher efficiency and wider view angle, compared to liquid crystal displays. In addition, because displays using organic electroluminescence phenomenon permit a total module thickness of 2 mm or less and can be manufactured on plastic substrates having a thickness of 0.3 mm or less, it is possible to meet the trend of thinning and downsizing of displays. Moreover, organic electroluminescence displays have an additional advantage in that they are produced at a lower cost compared to liquid crystal displays.
Organic light emitting devices are based on the mechanism wherein electrons and holes injected to an organic film formed of organic compounds through an anode and a cathode form exitons when they are recombined and then light having a certain wavelength is emitted from the exitons. In 1965, Pope et al. found electroluminescence in an anthracene single crystal for the first time. Following this, in 1987, Tang et al. in Kodak Co. found that an organic light emitting device formed of organic materials with a structure having separate functional laminated layers, i.e., a hole transport layer and light emitting layer laminated to each other, can provide a high luminance of 1000 cd/m2 or higher even under a low voltage of 10V or less. After those findings, organic light emitting devices has been a matter of great interest in the field of display technology (Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913). Such organic light emitting devices are classified into those using fluorescence and those using phosphorescence capable of providing a high efficiency of up to three times of the fluorescence-based efficiency. Alternatively, such organic light emitting devices may be classified according to molecular weights of the organic materials forming organic light emitting devices, i.e., those prepared by a low-molecular weight method wherein a device is formed by using a vacuum sublimation process and those prepared by a high-molecular weight method wherein a device is formed by using solution processes such as a spin coating, ink jet printing or roll coating process.
As shown in FIG. 1, a conventional organic light emitting device includes an anode, a hole injection layer that accepts holes from the anode, a hole transport layer that transports holes, a light emitting layer in which holes and electrons are recombined to emit light, an electron transport layer that accepts electrons from a cathode and transport them to the light emitting layer, and a cathode. The above thin film layers are formed by a vacuum deposition process. The reason for manufacturing organic light emitting devices having a multilayered thin film structure is as follows. It is possible to transport holes and electrons to a light emitting layer more efficiently when a suitable hole transport layer and electron transport layer are used, because the moving rate of holes is significantly higher than that of electrons in organic materials. Additionally, it is possible to increase luminous efficiency when hole density is balanced with electron density in a light emitting layer.
Hereinafter, a conventional organic light emitting device will be explained referring to FIG. 1.
A substrate 1 is the support for an organic light emitting device and may be formed of a silicone wafer, quartz or glass plate, metal plate, plastic film or sheet, etc. Preferably, glass plates or transparent plates made of synthetic resins such as polyester, polymethacrylate or polysulfone are used.
A first electrode (anode) 2 is disposed on the substrate 1. The anode serves to inject holes to a hole injection layer 3 and may be formed of metals such as aluminum, gold, silver, nickel, palladium or platinum, metal oxides such as indium-tin oxides or indium-zinc oxides, halogenated metals, carbon black, or conductive polymers such as poly(3-methylthiophene), polypyrrole or polyaniline.
The hole injection layer 3 is disposed on the anode 2. Materials used in the hole injection layer have to provide high efficiency of hole injection from the anode and have to transport the injected holes efficiently. In this regard, the materials should have low ionization potential, high transparency to visible light and excellent stability to holes.
Materials for the hole injection layer include compounds that have excellent thermal stability while maintaining a stable interface with the anode. Typical examples of the materials include copper phthalocyanine (CuPc), which is a porphyrin-copper complex disclosed in U.S. Pat. No. 4,356,429 by Kodak, Co. Because CuPc is the most stable compound for use in a hole injection layer, it has been used widely. However, it shows an absorption band at the blue and red zones, and thus has problems when manufacturing full-color display devices. Recently, starburst-like aromatic aryl amine compounds having no absorption band at the blue zone are known (U.S. Pat. No. 5,256,945 and Japanese Laid-Open Patent No. 1999-219788, and see the following formulae 4-12). Particularly, among the starburst-like amines having no absorption band at the blue zone, compounds represented by the following formulae 8-12 having a glass transition temperature of 100° C. or higher and excellent stability are used.

Recently, many hole injection materials having a higher glass transition temperature and more improved thermal stability have been reported. Most of them are compounds derived from NPB of Kodak, Co. and are represented by the following formulae 13-17 (see, Japanese Laid-Open Patent No. Hei9-301934 and U.S. Pat. Nos. 6,334,283 and 6,541,129).

Additionally, Japanese Laid-Open Patent No. 2003-238501 discloses aromatic oligoamine derivatives having at least five nitrogen atoms in one molecule (formulae 18 and 19).

Further, more recently, Japanese Laid-Open Patent No. 2003-317966 and U.S. Pat. No. 6,660,410 disclose a carbazole group-containing material (formula 20), which is specifically used as host forming a light emitting layer in an organic light emitting device using phosphorescence and is claimed to improve the lifespan of an organic light emitting device compared to conventionally known CBP (carbazole biphenyl). Other compounds used in a hole injection layer are represented by the following formulae 21-27.

A hole transport layer 4 is disposed on the hole injection layer 3. The hole transport layer serves to accept holes from the hole injection layer and transport them to an organic light emitting layer 5 disposed thereon. The hole transport layer has high hole transportability and stability to holes. It also serves as a barrier to protect electrons. In addition to the above-mentioned basic requirements, when it is used in display devices for cars, for example, it is preferable that the materials for a hole transport layer have an improved heat resistance and a glass transition temperature (Tg) of 80° C. or higher. Materials satisfying such requirements include NPB, spyro-arylamine compounds, perylene-arylamine compounds, azacycloheptatriene compounds, bis(diphenylvinylphenyl)anthracene, silicon germanium oxide compounds, silicon-containing arylamine compounds, or the like.
Meanwhile, as an important organic single molecules for a hole transport layer, there is arylamine compounds having high hole transport rate and excellent electrical stability. In order to improve thermal stability of arylamine compounds, hole transport materials into which a naphthyl substituent or spyro group is introduced are reported (see, U.S. Pat. Nos. 5,554,459 and 5,840,217). In the beginning, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) is frequently used as organic hole transport material. However, because TPD is unstable at a temperature of 60° C. or higher, N-naphthyl-N-phenyl-1,1′-diphenyl-4,4′-diamine (NPD) based materials or amine compounds substituted with a greater number of aromatic groups that have a higher glass transition temperature are used at the present time. Particularly, organic single molecules for use in a hole transport layer should have high hole transport rate. Additionally, because a hole transport layer is in contact with a light emitting layer and forms an interface therebetween, organic single materials for a hole transport layer should have an adequate ionization potential value of between that of a hole injection layer and that of a light emitting layer so as to inhibit the generation of exitons at the interface between hole transport layer and light emitting layer. Further, the organic single materials for a hole transport layer are required to control the electrons transported from the light emitting layer.
An organic light emitting layer 5 is disposed on the hole transport layer 4. The organic light emitting layer, which serves to emit lights by the recombination of holes and electrons injected from the anode and cathode, respectively, is formed of materials having high quantum efficiency.
Organic single molecules for use in a light emitting layer where light emission is accomplished by the recombination of holes and electrons are classified functionally into host materials and guest materials. In general, host materials or guest materials can accomplish light emission when used alone. However, host materials are doped with guest materials in order to solve the problems of low efficiency and luminance and the problem of self-packing of the same molecules that causes the excimer characteristics to come out in addition to the unique characteristics of each molecule.
More particularly, as green light emitting layer, 8-hydroxyquinoline aluminum salt (Alq3) is uniquely used and may be doped with high-quantum efficiency materials such as quinacridone or C545t so as to increase luminous efficiency. Organic materials for a blue light emitting layer have problems in that they have low melting points and low luminous stability at the initial time and that they have poor lifespan, compared to Alq3 as green light emitting material. Additionally, because most materials for a blue light emitting layer represent a light blue color rather than pure blue color, they are not suitable for full-color version displays, and so, they are also doped with perylene or distryl amines (DSA) to increase luminous efficiency. Typical organic materials for a blue light emitting layer include aromatic hydrocarbons, spyro-type compounds, aluminum-containing organometallic compounds, heterocyclic compounds having an imidazole group, fused aromatic compounds, as disclosed in U.S. Pat. Nos. 5,516,577, 5,366,811, 5,840,217, 5,150,006 and 5,645,948. Meanwhile, in the case of a red light emitting layer, a large amount of green light emitting material doped with a small amount of red light emitting material is used due to the characteristically narrow band gap of red light emission. However, such materials have structural problems disturbing the improvement of lifespan.
An electron transport layer 6 is disposed on the organic light emitting layer 5. In the electron transport layer 6, such materials as having high electron injection efficiency from a cathode 7 (a second electrode) and capable of transporting the injected electrons efficiently are used. For satisfying this, the materials should have high electron affinity and electron moving rate and excellent stability to electrons. Materials that meet the above requirements include: aromatic compounds such as tetraphenylbutadiene (Japanese Laid-Open Patent No. Sho57-51781), metal complexes such as 8-hydroxyquinoline aluminum (Japanese Laid-Open Patent No. Sho59-194393), metal complexes of 10-hydroxybenzo[h]quinoline (Japanese Laid-Open Patent No. Hei6-322362), cyclopentadiene derivatives (Japanese Laid-Open Patent No. Hei2-289675), bisstyrylbenzene derivatives (Japanese Laid-Open Patent Nos. Hei1-245087 and Hei2-222484), perylene derivatives (Japanese Laid-Open Patent Nos. Hei2-189890 and Hei3-791), p-phenylene derivatives (Japanese Laid-Open Patent Nos. Hei3-33183 and Hei11-345686), oxazole derivatives, or the like.
Additionally, preferred organic single molecules for use in an electron transport layer include organometal complexes having relatively high stability to electrons and high electron moving rate. Particularly, it is reported that Alq3 is the most preferred, because it has excellent stability and high electron affinity. In addition to the above-mentioned materials, other electron transport materials known to one skilled in the art include Flavon or silol series available from Chisso Corporation.
There is no especially preferred candidate other than the above materials for use in the electron transport layer. Generally, electron transport materials are used in the form of a mixture with metals for use in cathodes. Otherwise, inorganic materials such as lithium fluoride (LiF) may be used.
The cathode 7 serves to inject electrons to the organic light emitting layer 5. As materials for the cathode, the materials used in the anode 2 may be used. However, it is preferable to use metals having low work function in order to inject electrons more efficiently. Particular examples of the metals include lithium, cesium, sodium, tin, magnesium, indium, calcium, aluminum, etc., and alloys thereof.
However, the organic electroluminescence display device using organic single molecules suitable for each of the layers forming the device generally has short life span and has problems that it provides poor shelf durability and reliability. It is thought that such problems result from physical, chemical, photochemical and electrochemical changes in organic materials, oxidation of cathode, interlayer separation, and melting, crystallization and pyrolysis of organic compounds.